The surface of aerospace skin sheet is important for a variety of aerospace reasons. As is evident from a visual inspection of the different member airlines of the airline industry, some airlines prefer to paint the exterior of their airplanes, others prefer an unpainted surface, and some prefer both.
With an unpainted surface the aluminum may oxidize somewhat but there are certain disadvantages to painting an aircraft. One such disadvantage of painting the exterior of the airplane is that it adds weight. Paint may add as much as 200 to 700 pounds to the overall weight, dependent upon the size of the aircraft. Adding weight to an airplane affects load capacity, fuel capacity, additional maintenance, which funnels into diseconomies for the airline industry.
There are additional problems in painting the surface. While in the first instance the paint decorates and protects the surface, paint can also entrap moisture between the paint and aluminum layer promoting corrosion of the underlying aluminum surface and peeling of the paint. The friction of the air as it passes over the aircraft will deteriorate a painted surface, add additional drag, and decrease the economic benefits.
Pristine surfaces of aluminum have less drag when the aircraft is in use since the smoothness of the surface provides less frictional resistance. An unpainted aluminum surface also provides a lighter aircraft, which provides a weight and fuel advantage thus resulting in a more economical in-service aircraft.
Polishing the surface of aerospace skin sheet has been a labor intensive, time consuming, and costly process. One of the major defects, among others, in aluminum and aluminum alloy surfaces is a phenomenon called "orange peel", which visually can look like a starry sky when light is reflected. While it is not exactly known how orange peel is created on the surface of the aluminum, although many theories exist, it is known that it is undesirable by both those airlines that do not paint their aircraft and those that do. Consequently, to arrive at an acceptable surface quality the surface of the aluminum must be treated to rid itself of this scourge. Unless treated in a rigorous manner, the orange peel remains making the piece of skin sheet economically unacceptable. Unacceptable aluminum skin sheet in the aerospace aluminum and aluminum alloy art of today is also known as scrap.
Another problem in aluminum surfaces are defects called blemishes. Blemishes can occur from a variety of incidences, such as defects incurred during the rolling process or during shipping, storing, handling, and ordinary day to day treatment of inventoried skin sheet. These blemishes are also unacceptable under aerospace customer specifications and either must be fixed or the sheet may be scrapped.
Recovery rate for blemished skin sheet may be defined as the capability to take blemished skin sheet and turn it into a commercial sheet that the aerospace industry would readily accept. The recovery rate of blemished sheets from either orange peel and/or blemishes is historically very low.
In its ordinary workings, to arrive at an acceptable polished aluminum and/or aluminum alloy surface it can take as much as 2 to 3 hours of rigorous treatment of the surface of the skin sheet. At the outset, polishing the surface of the skin sheet under the prior art requires the single step of placing the sheet in an apparatus called a polishing machine. These machines are generally very large, expensive, and capital intense. The machines are usually equipped with a polishing means, such as a soft brush roll. Once the sheet has been lifted into place on a part of the machine where it can be held in a flat position the sheet is movably brushed against the roll to thereby polish or buff. One sweep back and forth on the length of the entire sheet counts as one pass. It may take as many as 40 passes for a sheet to become acceptable and thereby meet the required aerospace specification. Typically, if at 40 passes a surface inspector determines that the sheet will not meet the specification, the piece will be scrapped. If not scrapped the sheet is then passed on to inventory to await customer delivery.
Repairing blemished sheets, on the other hand, is a process that has evolved over the years into a complex array of steps and processes that are lengthy and costly. Often a repair person must spend hours on hands and knees sanding and/or burnishing, and then polishing an isolated blemish. For example, defects known as comet tails are visually apparent upon surface visual inspection, and while isolated on the surface can number in the hundreds on a single sheet from time to time. As the name evokes, comet tails start as a surface inclusion the can produce a considerable gouge with an appended comet tail. Comet tails can occur during polishing or prior thereto but become apparent during polishing. Other defects also become apparent during polishing and in order to correct the defect, the sheet must be taken off the polishing machine and hand sanded, hand polished, then subsequently placed on the polishing machine again to blend the hand polish with the machine polish to produce a homogeneous surface polish.
Repair activities incur an additional disadvantage due to inherent ergonomic challenges. The repetitive hand movements and hand rotations required to polish the sheet require a worker to use certain motions that caused a variety of ailments. Such ailments lead to worker injury and absence, let alone the misery of the ailment itself.
The invention hereof, has solved many of these problems in the prior art. The invention requires a significantly decreased number of polishing passes and consequently has decreased the time entailed in the polishing activity. The inventive process also has decreased the onerous process involved in the repair activity, decreasing the time to effect that repair and decreasing the time to polish the repair with a resultant decrease in ergonomic injuries.